A plasma ion source mass spectrometer of the prior art is described in Items 22 to 25 of Measurement Series 28 "Plasma Ion Source Mass Spectrometry", the Spectroscopical Society of Japan, edited by Koji Kawaguchi and Taketoshi Nakahara. In this description, object elements contained in a sample solution are ionized by a plasma, and the resultant ions are extracted through an interface unit into a vacuum so that they may be measured by a mass filter. For extracting the ions produced by the plasma into the mass filter, the interface unit generally uses a conical sampling cone and skimmer cone. This interface unit, especially the sampling cone is required to endure the high temperature of the plasma because it is exposed directly to the plasma whose temperature reaches as high as 5,000 to 6,000.degree. C. For this requirement, the sampling cone is usually made of a material having an excellent thermal conductivity and is sufficiently cooled with cooling water so that it may not melt down.
However, the interface unit is given a role to introduce the object elements contained in the solution sample and ionized by the plasma into the vacuum in the same state as in the plasma. If a portion is cooled with the cooling water to a lower temperature, a molecular ion peak may be formed by the cooling, or the interface unit surface may be coated with insulator and charged up by the molecular ion peak formed. This causes a problem of a drop of sensitivity.
As disclosed in Japanese Patent Laid-Open No. 161335/1995, therefore, the temperature of an orifice formed in the sampling cone of the interface unit is detected by means of an optical temperature sensor, and the high-frequency electric power to be supplied to the cavity for generating the plasma is adjusted so that the temperature of the orifice may not allow recombination of ions.
However, this method of adjusting the high-frequency electric power to be supplied to the cavity changes the condition of the plasma generation, so that the ionizing condition is changed to bring up a problem of the change in the analysis sensitivity.